Tin dioxide was the first material used to fabricate gas sensors, though many other metal oxides have been employed since. Doping the gas sensitive layer with a noble metal and the application of thin film technology allows a significant improvement in performance of these devices due to the catalytic effect of the metal on reactions between the oxide and adsorbed gas molecules and the formation of a Schottky barrier. The microstructure of the metal oxide also plays a fundamental role and systematic studies of the relationship between deposition conditions and film structure are therefore essential to understand the sensor response. A low deposition temperature leads to a porous granular structure, while for a higher temperature the greater atomic mobility results in more ordered columnar structures. This latter type of structure increases the efficiency of gas detection because the intercolumnar interfaces facilitate diffusion of gas molecules. The sensitivity is also increased by reducing the grain size of the metal oxide films to ~10nm, so that the depth of the depleted region becomes comparable with the grain diameter. Two commonly used techniques for the production of metal oxide thin films, reactive r.f. sputtering and pulsed laser deposition, are compared. The application of microelectronic fabrication techniques combined with thin film synthesis methods is being used to develop integrated CMOS compatible microsensor arrays. While thin film sensors are gradually supplanting thick film ones, thick film technology is still used, however, in the majority of commercial gas sensors. Extensive field testing of air quality monitoring systems based on metal oxide sensors has been carried out to compare their performance with conventional analytical instruments. Further improvements in sensitivity will be possible due to the development of novel oxide architectures such as nanowires and nanobelts, which have a much larger surface area in contact with the gas molecules, and the integration of these structures in miniaturised integrated devices. Solid state sensor systems together with advanced signal processing and data transmission via the existing telecommunications infrastructure will in future allow the implementation of intelligent sensor networks to provide information on air pollution with high spatial and temporal resolution.